Assassin ends up marrying the priest to get citizenship, a passport, and status as Benika's "mommy".
(Yes, Catholic priests aren't supposed to marry, but not like it's illegal in civil law.)
That's a shitty theory unless you saw the raws.
My theory is when they meet, the priest would say something like "Rose, my daughter. You may not be a child of god, but surely you're a child of peace." Then pulls out derringers and shoot her.